Design and manufacturing of a passive prosthetic knee joint

Passive prosthetic knee joints remain an attractive solution for transfemoral amputees in low-resource settings because of their low cost, mechanical simplicity, and ease of maintenance. But there are numerous commercial designs which are either relatively heavy or mechanically complex. In this work, the design, finite element analysis, material selection and fabrication of a lightweight passive prosthetic knee joint with a simple polycentric mechanism are proposed. Three candidate materials (AISI 4130 steel, Al2024-T4 and Al7075-T651) were selected and experimentally characterized by tensile testing and numerically compared in terms of stress distribution, deformation, weight and factor of safety and subjected to an extreme static loading of 1000 N. The maximum of von Mises stress obtained from the finite element calculations for all materials studied was 40.2 MPa, which was found to be below their respective yield strengths, thus providing sufficient structural integrity. Al7075-T651 was found to have the highest factor of safety (12.19), minimum structural deformation (0.0497 mm) and a good strength-to-weight ratio, thus showing the best overall performance among the materials evaluated, and the least prototype weight (198 g). The prototype weight was measured and compared with the CAD weight prediction (195 g), and good agreement was obtained with the fabricated prototype (198 g), with a deviation of 1.5%. This agreement indicates that the overall geometry, volume and assigned material density are consistent in the CAD model. The entire prototype was produced at an estimated cost of about USD 160, which is considerably less expensive than many passive prosthetic knee joints on the market. This proposed design is lightweight, has sufficient structural strength, a simple mechanical structure, and a low manufacturing cost, which make it a promising alternative for making transfemoral prostheses, especially in developing and resource-limited countries.